As information technology is developed rapidly and a desire for the communication of a human being is increased, mobile communication devices such as a portable device are necessities of a contemporary man. However, the effect of the electromagnetic wave generated by the device to a human body has been being an important issue as the use of the portable device is increased. At present, a relation between the electromagnetic wave in a frequency bandwidth used in a cellular phone and the effect of the electromagnetic wave to the human body has not been disclosed clearly. However, it has been being reported that the electromagnetic can affect various diseases such as a leukemia, an encephaloma, a headache, a decreased visual acuity, a brain wave disorder when the electromagnetic wave is accumulated in the human body, the damage of the reproduction ability of a man, etc.
In addition, a malfunction between information communication devices due to undesirable electromagnetic wave is constantly reported. This is an electromagnetic interference/electromagnetic compatibility (EMI/EMC) problem. Thus, the measurement method and the acceptable standard of the unwanted frequency of an electrical and an electronic communication device are prescribed in order to prevent interference between the devices due to the unwanted frequency. Moreover, the EMC should be considered at the stage of design and manufacture to satisfy the method and the standard and the EMC test should be satisfied for selling a product.
An electromagnetic wave reverberation chamber as an experiment facility for measuring the EMI and a radiation tolerance has been reported as the experiment result of a national institute standards and technology (NIST) and an international special committee on Radio interference (CISPR) prescribed specifications for the electromagnetic wave reverberation chamber in a IEC 61000-4-21.
The electromagnetic wave reverberation chamber is a measuring chamber of which all inner walls do not absorb the electromagnetic wave so that the electromagnetic wave has optimal reverberation time and diffusivity in the chamber in contrast to an anechoic chamber. The electromagnetic wave reverberation chamber satisfies a condition that the intensity of the electromagnetic wave is uniform inside the chamber and the electromagnetic wave is propagated in all directions uniformly in the any position inside the chamber. Thus, the electromagnetic wave anechoic chamber should absorb electromagnetic wave fully by using an electromagnetic wave absorber installed on the all inner walls of the electromagnetic wave anechoic chamber. However, the electromagnetic wave reverberation chamber does not need the absorber because the all inner walls of the chamber should reflect the electromagnetic wave fully. Instead, the electromagnetic wave reverberation chamber is generally manufactured by installing a metal wall on the inner wall thereof. The electromagnetic wave reverberation chamber obtains the uniformity of the electric field by using a stirrer in order to lower lowest usable high frequency (LUF). The uniformity of the electric field can be determined by a total number of modes which can be generated in the electromagnetic wave reverberation chamber, Q-factor of the medium used for manufacturing the electromagnetic wave reverberation chamber and the efficiency of the stirrer, etc.
Meanwhile, a technology improving the performance of the electromagnetic wave reverberation chamber by using the electromagnetic wave absorber has been reported.
FIG. 1 shows an outer shape of a conventional electromagnetic wave reverberation chamber and FIG. 2 depicts an inner shape of a conventional electromagnetic wave reverberation chamber.
As shown in FIGS. 1 and 2, the electromagnetic wave reverberation chamber can be manufactured in the shape of a polyhedron. The electromagnetic wave reverberation chamber has the shape of a cube generally. All the inner wall of the chamber consists of a metal conductor for the total reflection of the electromagnetic wave, and the stirrer and a pyramidal electromagnetic wave absorber are installed in the intended space for controlling the reflection characteristic of the inside of the chamber. The pyramidal electromagnetic wave absorber improves the reflection characteristic of the inside of the chamber so that more uniform electrical field distribution can be obtained. The position, size and electromagnetic field absorption rate of the pyramidal electromagnetic wave absorber affects the total performance of the chamber.
However, the conventional pyramidal electromagnetic wave absorber is not suitable for a device to be tested since the size of the conventional pyramidal electromagnetic wave absorber is so large that the inside space of the chamber can be small. In this case, since the electromagnetic wave reverberation chamber should be manufactured to be larger, there exist problems that the manufacture cost of the electromagnetic wave reverberation chamber is raised and an installation space for the chamber should be larger.
In addition, since the conventional pyramidal electromagnetic wave absorber is manufactured by using a material having absorption characteristic and is developed by a trial and error method, there exist considerable problems that the manufacturing process of the absorber is complicated and it is difficult to adjust the absorption characteristic and a absorption frequency bandwidth.
Meanwhile, as the additional examples of the conventional electromagnetic wave absorber, there is a 4/λ type wave absorber or a flat-plate type resonant absorber such as a Salisbury screen.
The construction of the resonant absorber is simple since the resonant absorber consists of a resistive film, a dielectric spacer and a metal conductor ground surface. Thus, the resonant absorber can be manufactured easily and the absorption performance thereof can be easily adjusted. In addition, when the resonant absorber is manufactured in a multilayer form, a multiple bandwidth absorption characteristic can be obtained.
However, the conventional resonant absorber has a problem that the thickness of the dielectric spacer should be equal to or larger than 4/λ from the metal conductor ground surface.